Unsafe Map overload spoils direct property access on parenthesized assignment expression

まだ誰も着手していません。

### 🔎 Search Terms

map overload any unknown unsafe weakmap set binary assignment lazy init

### 🕗 Version & Regression Information

- This is the behavior in every version I tried

### ⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.9.0-dev.20250326#code/DYUwLgBAxghlAWIBcECyMAOAeAzmATgJYB2A5gDQQD2ARgFYhRgB8EAPhAK7EAmIAZiRA8A3ACgx-bk0JViEMCDwBGABQBrEAE8UeImQCUEAN5iI0OXgj4lnYJAC8EVbAQgIAfg9PiIAO5omKoGBgB0pOAa2gbi5gD0cebmAHoeZtbgnPjyNjh2YOIAvhJSxDJyCkpgAExROhB6JKRGpuauiJ7eEL4B6BjBsRbEVrn5EE7tIOGRmlox6QlJEKnpNmBZObb2RWJAA

### 💻 Code

```ts
let cache: Map | undefined;

function test1(key: string) {
const result = (cache ??= new Map()).get(key);
// ^? const result: any
return result;
}

function test2(key: string) {
cache ??= new Map();
const result = cache.get(key);
// ^? const result: object | undefined
return result;
}
```

### 🙁 Actual behavior

First `result` gets spoiled as `any`

### 🙂 Expected behavior

I'd expect both `result`s to be `object | undefined`

### Additional information about the issue

This bug reports an issue related to `Map` but, tbf, it's not unique to `Map` and a more general solution would have to be found to properly fix this. The problem is how `checkBinaryLikeExpressionWorker` create a union of left and right types here - without taking into consideration that the assignment target might already overlap with the right type. That union type gets reduced with `UnionReduction.Subtype` and `.get` gets read from it - instead of the assigned to reference on the left side.

リンクされた TypeScript Playground を使って test1 と test2 の違いを再現し、続いて、レポートで代入オペランドの union を形成すると特定されている checkBinaryLikeExpressionWorker を調査します。.get が解決される前に、その union がどのように縮約されるかを追跡してください。括弧で囲まれた代入へのアクセスが、個別の代入形式と同様に object | undefined を推論し、報告された例に対するリグレッションカバレッジが追加されれば完了です。

索引モデルが issue の本文から書いたものです。